This invention relates generally to the field of electrolytic capacitors that are hermetically sealed, and in particular to a capacitor having a liquid-tight seal between the electrolyte solution and the terminal used to make an electrical connection between the capacitor and an external device.
Capacitors containing an electrolyte solution are subject to failure caused by leakage of the electrolyte liquid or vapor. For example, it is common for gas, such as hydrogen, to be evolved during operation, causing pressure to build inside the capacitor. Consequently, leaks may occur around conventional non-hermetic polymeric seals, where terminal wires protrude from the capacitor casing. To avoid this, a gas-tight hermetic seal is required.
One prior art solution has been to provide a hermetic, outer-metal/glass/inner-metal seal between the capacitor casing and the terminal wire. For the sake of clarity, the casing, typically the lid, is referred to as the “outer-metal” component, and the electrically conducting post, which is insulated by the glass, is referred to as the “inner-metal” component. Typically, the hermetic seal is positioned in an orifice, created in the lid of the casing. In the situation of an aluminum case, it has not proven economical, however, to provide a hermetic, aluminum to glass seal, due in part to the significant difference in the thermal coefficient of expansion of the glass or ceramic material used to construct the seal and the thermal coefficient of expansion of aluminum. Consequently, the hermetic seals used in capacitors are generally made with a metal other than aluminum, for example, steels (stainless or other alloys) or tantalum. Although the outer-metal portion of the seal is at approximately the same potential as the electrolyte, in general, the outer metal is integral with or welded to the case or lid metal, and should be of the same material in order to avoid galvanic corrosion if this region is exposed to the ionically conducting electrolyte.
To avoid galvanic corrosion of the inner-metal portion of the hermetic seal, a liquid-tight seal is typically utilized to prevent exposure of the inner region of the hermetic seal to the electrolyte. Even in the most optimum situation in which all components along the electrically conducting path between the inner-metal portion of the hermetic seal and the anode of the capacitor element consist of the same “valve metal”, so that galvanic corrosion is not an issue due to a dissimilar metal junction, a liquid-tight seal is still used to prevent the electrolyte from making contact with any portion of this conductive path. A “valve metal” is defined as a metal which grows an electrically insulating oxide in the presence of an electrolyte when a positive potential is applied to the metal with respect to the electrolyte. Examples of such metals are aluminum, tantalum, niobium, tungsten, titanium, zirconium. The two primary reasons that the inner portion of the hermetic seal, including a valve metal seal, should be protected from the electrolyte are the possibilities of intermetallics (impurities) in the valve metal that may not form a proper electrically insulating oxide in the presence of an electrolyte and/or insufficient creepage distance across the glass portion of the seal. Either of these two situations could result in undesirable electrical current flow between the high electrical potential of the inner-metal and the low potential of the outer-metal if the electrolyte is allowed access to all regions of the hermetic seal.
Various capacitor constructions have been disclosed to protect the components of the hermetic seal from corrosion. Sloan, U.S. Pat. No. 3,289,051, discloses a threaded cap containing a hermetic seal, which is screwed on the lid of the capacitor to compress a stack of bushing members. The apparatus of Sloan is complex to manufacture, requiring the assembly of numerous components, many of which must be welded together to maintain the hermeticity of the capacitor.
In U.S. Pat. No. 4,987,519, Hutchins et al. disclose a cylindrical capacitor with a seal created by crimping. An inwardly directed annular bead is formed, which presses an O-ring into a plastic bushing. While the foregoing technique has found utility with cylindrical capacitors, it is not effective for sealing capacitors having other geometric configurations, such as a rectangular prism. Additionally, the capacitor disclosed in U.S. Pat. No. 4,987,519 requires a second seal where the riser wire protrudes from the plastic bushing.
Capacitors having a non-cylindrical casing, especially capacitors having a casing with a flat surface, are particularly difficult to seal. Prior art methods of sealing the liquid typically employ a gasket or seal around the inside perimeter of the casing. As gas pressure builds inside the capacitor, the flat surface may bulge outward, creating a gap between the O-ring or gasket and the casing. Electrolyte can seep through the gap and corrode the terminal in the hermetic seal.